11:15 AM - 11:30 AM
[HQR04-09] Eventual sediments presumed in lake deposits of coastal lakes along the western coast of Kyushu Island, Japan
Keywords:The Holocene, Climatic Change, Hazard Deposits, Coastal lakes, Kyushu Island, Diatom
In coastal lake deposits often contained several disturbed layers by past tsunami, typhoons, high tide and natural hazard events. Those eventual layers were well preserved among lake sediments which deposited under stable water conditions, were identified easily by lithofacies of the sediments. In this paper, we presumed paleo-environmental histories of those eventual layers by diatom assemblages in the sediments. The compositions of diatom assemblages were shifted by characteristics and origins of natural disasters. Along the western coast of Kyushu Island where very huge typhoons often attacked by historic documents and meteorological records, we took undisturbed lake cores at coastal lakes to presume the frequencies and historic records of the disasters. Our research areas were (a) Ikeda-ike (Daijya-ike) at Amakusa Peninsula, (b) Nakayama area at Shimokoshiki Island, (c) Nakakoshiki area at Nakakoshiki Island and (d) Namako-ike at Kamikoshiki Island.
(a) Ikeda-ike (Daijya-ike) at Amakusa Peninsula: It was a small lake (WE200m, NS 150m), and has continued freshwater environment since 6700 yBP when the sand barrier closed the oceanic side of the lake. We presumed 11 eventual layers in the core sediments. Diatom assemblages presumed six of them were hazards with clear marine water invasions into the lake, two of them were with unclear marine invasions and last three were caused by huge flooding from surrounding mountains.
(b)Nakayama area at Shimokoshiki Island: It was a freshwater lake, however it changed to be a reclamation rice field about 50 years ago. The ancient lake had continued freshwater environment since 6500 yBP. Totally 16 eventually layers were presumed in the core. Diatom assemblages presumed seven of them were caused by marine invasions, and six of them were caused by inland flooding without marine invasions. Two of them were characterized by water level increasing (maybe artificially) and the last one was caused by the artificial land reclamation.
(c) Nakakoshiki area at Nakakoshiki Island: The top part of the core was disturbed by recent land flooding, the sediment during 2000-6000 yBP was kept. Four eventually layers were observed, and three of them were caused by marine invasions. The other one was caused by inland flooding without marine invasions.
(d) Namako-ike at Kamikoshiki Island: It is a brackish lake along north east coast of Kamikoshiki Island. We took the core until 6200 yBP layers, and 9 eventually layers were observed. Six of them were caused by caused by marine invasions, and three of them were characterized by freshwater flooding into the lake
(e) Correlation of the eventual layers in the cores: We found the strong correlation between the eventual layers at the research sites. The frequencies of the hazards surrounding 2.4ka, 2.8ka, 3.2ka, 3.7ka, 4.2ka, 4.7ka and 5.2ka were increased. They could be correlated to the hazard layers of Korean Peninsula, and were probably related to the global climatic processes in East Asia.
(a) Ikeda-ike (Daijya-ike) at Amakusa Peninsula: It was a small lake (WE200m, NS 150m), and has continued freshwater environment since 6700 yBP when the sand barrier closed the oceanic side of the lake. We presumed 11 eventual layers in the core sediments. Diatom assemblages presumed six of them were hazards with clear marine water invasions into the lake, two of them were with unclear marine invasions and last three were caused by huge flooding from surrounding mountains.
(b)Nakayama area at Shimokoshiki Island: It was a freshwater lake, however it changed to be a reclamation rice field about 50 years ago. The ancient lake had continued freshwater environment since 6500 yBP. Totally 16 eventually layers were presumed in the core. Diatom assemblages presumed seven of them were caused by marine invasions, and six of them were caused by inland flooding without marine invasions. Two of them were characterized by water level increasing (maybe artificially) and the last one was caused by the artificial land reclamation.
(c) Nakakoshiki area at Nakakoshiki Island: The top part of the core was disturbed by recent land flooding, the sediment during 2000-6000 yBP was kept. Four eventually layers were observed, and three of them were caused by marine invasions. The other one was caused by inland flooding without marine invasions.
(d) Namako-ike at Kamikoshiki Island: It is a brackish lake along north east coast of Kamikoshiki Island. We took the core until 6200 yBP layers, and 9 eventually layers were observed. Six of them were caused by caused by marine invasions, and three of them were characterized by freshwater flooding into the lake
(e) Correlation of the eventual layers in the cores: We found the strong correlation between the eventual layers at the research sites. The frequencies of the hazards surrounding 2.4ka, 2.8ka, 3.2ka, 3.7ka, 4.2ka, 4.7ka and 5.2ka were increased. They could be correlated to the hazard layers of Korean Peninsula, and were probably related to the global climatic processes in East Asia.